1. Field of the Invention
This invention relates to a process for preparing a low molecular weight organosiloxane terminated with a silanol group.
2. Prior Art
Linear organopolysiloxanes having a hydroxyl group at either end are dispersants useful in preparing silicone rubber compounds. Various dispersants are used while their dispersing function is proportional to the content of hydroxyl group. Then polysiloxanes having a higher content of hydroxyl group, that is, low molecular weight linear organosiloxanes terminated with a hydroxyl group are effective in less amounts, which is advantageous for the subsequent processing of silicone rubber compounds. However, currently commercially available dispersants are linear organopolysiloxanes of not less than organotetra-siloxanes. For preparing potentially more effective organosilanes and siloxanes having a silanol group such as monomer diols, dimer diols and trimer diols, few processes are industrially practical.
Research efforts have been made on the synthesis of short chain hydroxysilanes and siloxanes. A method of effecting hydrolysis of an alkoxysilane solution while maintaining it neutral using a buffer solution or the like is known although this method is for laboratory use and not practical on an industrial scale. Another known method involves mixing dimethoxysilane with an excess of neutral distilled water and refluxing the mixture, but the end product is obtained in low yields.
Low molecular weight linear organopolysiloxanes terminated with a hydroxyl group are industrially prepared by effecting hydrolysis of linear organochlorosiloxanes having a chlorine atom at each end or chlorosilanes in a weakly alkaline aqueous solution under controlled conditions to avoid formation of cyclics. This method, however, has the problem that since the silanol group is unstable to acid or alkali, HCl resulting from hydrolysis can induce condensation reaction to form higher molecular weight organopolysiloxanes and cyclic polysiloxanes as well as the desired organopolysiloxanes. Therefore, this method is difficult to synthesize the desired linear organopolysiloxanes unless a strict control is made to maintain hydrolyzing water neutral.
In another known method, organochlorosiloxanes are reacted with acetic acid to convert into acetoxy form which is hydrolyzed. Since hydrolysis does not proceed to completion, the end product contains some residual acetoxy groups and is thus unsuitable as a dispersant to be used in silicone rubber.
U.S. Pat. No. 3,925,285 discloses a method for synthesizing a low molecular weight linear organopolysiloxane having a silanol terminal group by reacting hexamethylcyclotrisiloxane, methanol, formic acid and water, the resulting organopolysiloxane having some methoxy groups left therein. This method is costly since the starting reactant, hexamethylcyclotrisiloxane is relatively expensive. There is not produced a low molecular weight linear organopolysiloxane having less than three D units ((CH.sub.3).sub.2 SiO) and a silanol terminal group. From a reaction aspect, there is not produced a product which is shorter than 1,5-dihydroxytrisiloxane. The content of hydroxyl group has a certain limit.
U.S. Pat. No. 5,057,620 discloses that a chlorosiloxane is added dropwise to a water-containing epoxy solvent such as propylene oxide and butylene oxide. This method also uses relatively expensive hexamethylcyclotrisiloxane and low boiling solvents have the safety problem of electrostatic ignition.
Japanese Patent Publication (JP-B) No. 5604/1989 discloses a method for synthesizing a short chain silanol by hydrolyzing an alkoxysilane in the presence of a solid acid catalyst such as activated clay. This method requires complex operation since the solid acid must be neutralized. The product of this method is a mixture of short chain silanols with the yield of dimer diol being about 50%. With respect to monomer diol, it is obtained at a purity of 84 molt in a yield of less than 10% as reported in Example 29.
Moreover, hydrolysis of an alkoxysilane in the presence of a cation exchange resin to produce a resin having a high degree of polymerization is disclosed in U.S. Pat. No. 3,304,318. No reference is made to the synthesis and yield of short chain silanols, especially dimer diol and trimer diol.